In certain types of cellular radio communication systems, such as one specified by EIA/TIA-553, a forward analog control channel (FOCC) is a continuous wideband data stream sent from a base station to a mobile terminal or station. This data stream is generated at a 10 kilobit/second .+-.0.1 bit/second rate. FIG. 1A depicts the format of the FOCC data stream.
Each forward analog control channel consists of three discrete information streams referred to as stream A, stream B, and a busy-idle stream. These three information streams are time-multiplexed together. Messages to mobile stations with the least significant bit of their mobile identification number equal to "0" are sent on stream A, and those with the least-significant bit of their mobile identification number equal to "1" are sent on stream B. Thus, a given mobile station monitors only stream A or stream B. The busy-idle stream contains busy-idle bits (shown as arrows in FIG. 1A) which are used to indicate the current status of the reverse control channel. The reverse control channel is busy if the busy-idle bit is equal to "0", and idle if the busy-idle bit is equal to "1". A busy-idle bit is located at the beginning of each dotting sequence, at the beginning of each word sync sequence, at the beginning of the first repeat of word A, and after every 10 message bits thereafter.
A 10-bit dotting sequence (1010101010) and an 11-bit sync sequence (11100010010) are sent to permit mobile stations to achieve synchronization with the incoming data. Each word contains 40 bits, including parity, and is repeated five times. The repeated word is referred to as a word block or as a frame. For a multi-word message, the second word block and subsequent word blocks are formed the same as the first word block, including the 10-bit dotting and 11-bit word synchronization sequences. A word is formed by encoding 28 content bits into a (40, 28) Bose-Chaudhuri-Hocquenghem (BCH) code that has a distance of 5, (40, 28; 5). The left-most bit (i.e., earliest in time) is designated as the most-significant bit. The 28 most-significant bits of the 40-bit field are defined to be the content bits. The generator polynomial for the (40, 28; 5) BCH code is: EQU g.sub.B (x)=X.sup.12 +X.sup.10 +X.sup.8 +X.sup.5 +X.sup.4 +X.sup.3 +X.sup.0.
The code, a shortened version of the primitive (63, 51; 5) BCH code, is a systematic linear block code with the leading bit as the most significant information bit and the least-significant bit as the last parity-check bit.
Each FOCC message can consist of one or more words, and can thus be transmitted over one or more frames. The types of messages transmitted over the forward control channel are mobile station control messages, overhead (OHD) messages, and control-filler messages.
A three-bit OHD field is used to identify the overhead message types. Overhead message type codes can be grouped into the following functional classes: a system parameter overhead message, a global action overhead message, a registration identification message, a control-filler message, and a digital control channel information message.
Overhead messages are sent in a group called an overhead message train (OMT). The first message of the train is the system parameter overhead message (SPOM). The desired global action messages or registration ID message or Digital Control Channel Information message are appended to the end of the system parameter overhead message. The total number of words in an overhead message train is one or more than the value of a NAWC field contained in the first word of the system parameter overhead message. The last word in the overhead message train is identified by a "1" in the END field of that word; the END field of all other words in the train is set to "0". For NAWC-counting purposes, any inserted control-filler messages are not counted as part of the overhead message train.
The system parameter overhead message (SPOM) is sent every 0.8.+-.0.3 seconds on each of the control channels. The global action message, registration identification message and Digital Control Channel Information message are sent on an as needed basis.
Referring now also to FIG. 1B, the control-filler message is defined to be one word having 28 data bits and 12 parity bits. It is sent whenever there is no other message to be sent on the forward control channel. It may be inserted between messages, as well as between word blocks (frames) of a multi-word message. The control-filler message is chosen so then when it is sent, the 11-bit word sync sequence (11100010010) will not appear in the message stream, independent of the busy-idle bit status.
The following table illustrates the format of the control-filler message as defined in the IS-136.2, Rev. 0, specification (May 17, 1995).
Information Element Length (bits) T.sub.1 T.sub.2 = 11 2 DCC 2 010111 6 CMAC 3 SDCC1 2 11 2 SDCC2 2 1 1 WFOM 1 1111 4 OHD-001 3 P 12
The interpretation of the data fields is as follows:
T.sub.1 T.sub.2 Type field, Set to 11 indicating overhead word PA1 DCC Digital color code field PA1 CMAC Control mobile attenuation field. Indicates the mobile station power level associated with the reverse control channel, and is used by the mobile station when accessing the system on the reverse control channel PA1 SDCC1, PA1 SDCC2 Supplementary Digital Color Codes. If the Supplementary Digital Color Code feature is utilized, the combination of SDCC1 and SDCC2 transmitted by the base station must be a non-zero number. Mobile stations which respond with a non-zero supporting SDCC combination are capable of supporting SDCC. Mobile stations which respond with a zero SDCC combination are not capable of supporting SDCC. The zero SDCC combination is used to indicate either that SDCC1 and SDCC2 are not used or are not supported. PA1 WFOM Wait-for-overhead-message field which indicates whether the mobile station must read an overhead message train (OMT) before accessing the system PA1 OHD Overhead Message Type field, Set to 001 for indicating the control-filler word PA1 P Parity field
The control-filler messages are generally not required unless the mobile station is powered up and/or if a call is to be made. However, typically 50% to 70% (or more) of received messages are control-filler messages. The exact number sent during any given period is a function of system loading. As more mobile stations are being serviced by the system fewer control-filler messages are transmitted, and vice versa.
As may be appreciated, the reception of the FOCC, or the Analog Control Channel (ACC) in a dual mode analog/digital system such as that specified in IS-136, consumes some amount of battery power of the mobile terminal. The conservation of battery power is an important goal in the design and operation of a mobile terminal, in particular a cellular radiotelephone.
Reference can be had to the following patents for teaching various aspects of power saving in a mobile terminal: commonly assigned U.S. Pat. Nos. 5,291,542, 5,396,653, and 5,613,235 by Raimo Kivari et al., and U.S. Pat. No. 5,471,655, by Raimo Kivari. Reference may also be had to U.S. Pat. Nos. 5,224,152, 5,175,874, 4,592,076 and 4,029,900.
For example, in U.S. Pat. No. 5,471,655 Kivari describes an extended standby (XSTBY) mode of operation for conserving battery power, while in U.S. Pat. No. 5,613,235 Kivari et al. describe a synchronous XSTBY mode of operation. In the standby mode of operation the mobile station monitors the FOCC or ACC for a paging message or for other information. Since the mobile station must be at least intermittently powered during the monitoring times, some battery power consumption occurs. These commonly assigned patents teach various techniques for reducing the battery drain, and thereby extending the time between battery rechargings.
In commonly assigned U.S. patent application Ser. No. 08/599,144, Feb. 9, 1996, entitled "Mobile Terminal Having Power Saving Mode That Monitors Specified Numbers of Filler Messages", by Raimo Kivari, there is described a technique to eliminate the reception of control-filler messages based at least in part on an improved messaging function wherein a mobile station is informed from the base station of a number of consecutive control-filler messages that will be transmitted.
In conventional implementations of the analog control channel (ACC) reception mode the standby time is significantly shorter than the standby time when operating in the digital mode with a digital control channel (DCCH). This is due to the fact that in the analog mode (ACC/Extended Standby, XSTBY) the mobile station RF circuitry is powered for a longer time (e.g., 16%-100%) as compared to operation in the digital mode (minimum about 1%).